The present invention relates to a method of treating surfaces, such as coating moving webs of paper, textile fabrics or rotating rollers, etc., with treating agent, the treating agent being in the form of a foam.
In the paper manufacturing field, for instance, various methods have long been used to give the paper certain desired properties. A coating agent in liquid form, for instance a clay dispersion in water, may be applied on a paper web and then smoothed over the surface of the paper to give the paper better properties than it would have had in its original, untreated state, thus making it more suitable for printing purposes, for example. Another example of a process which has been used for a long time to improve the properties of the paper is surface-sizing to give the surface of the paper a certain strength. This process comprises applying a water solution to one or both sides of the paper, in which solution for instance, a starch has been dissolved. A usual method of application is treatment in a size press consisting of two rollers rotating against each other and pressed together to form a pressure nip. The paper web to be treated is passed through this pressure nip and the starch solution is at the same time applied to both sides of the web before the pressure nip. By a suitable choice of the concentration of the solution and the pressure between the rollers, the amount of solution absorbed by the paper web can be regulated to a certain extent. The paper web, which has become wet during the treatment, is then allowed to pass a suitable drying means so that the water added can evaporate. By means of this process, therefore, the paper has been supplied with a certain quantity of starch and the presence of the water has been necessary in order to ensure homogeneous distribution of the starch in and on the paper web.
Usually only small quantities of the treating agent, for instance starch, are to be applied on the paper web during sizing or other surface treatment whereas liquid, for instance water, must be used in vast quantities. A great deal of water will therefore be absorbed by the paper web thus making the process uneconomic since considerable quantities of energy must be used to remove the water applied.
Only one method has been mentioned by way of introduction for coating moving webs of paper, for instance. There are, however, innumerable other methods and apparatus for surface treating paper, for instance. Instead of the size press described above, coating methods exist in which an excess of coating agent is applied on the paper web by rollers, slots or the like, after which the excess is scraped off by a flexible blade, rotating rod, air brush or the like. For the same reason as stated above in connection with the size press, there is a general endeavour to reduce the quantity of liquid in which the coating agent is dissolved or dispersed in order to save energy and drying equipment in the subsequent drying process. When clay-coating, therefore, that is to say treating a paper web with a coating agent such as clay and a binder dispersed in water, success has recently been achieved in producing a coated paper with a dryness content, i.e. the ratio of dry additives to the total weight of the coating agent, of up to 70 percent by weight. In these cases the weight of coating applied per unit area (calculated as dry) is relatively high. As an example it may be mentioned that clay-coated paper often has a coating of 10-20 gram dry weight per m.sup.2.
With respect to surface-treating paper, for instance, by surface-sizing, the desired quantities of dry treating agent are considerably lower. It has been found suitable, for instance, when sizing paper intended for offset printing, to use a quantity of binder in the form of starch or CMC which is less than one gram of dry binder per m.sup.2. In some cases a certain amount of binder should penetrate to a certain extent into the paper whereas in other cases only a surface coating is required. In many other cases also where it is desired to influence the function of the paper by surface treatment, the quantities of treating agent are extremely small. It may be a question of surface-colouring or a treatment to alter the barrier properties of the paper by treating it with synthetic polymers or waxes. Other examples include treating with friction-reducing agents or agents giving the paper release-properties and water-treatment of the surface to give the paper a certain moisture content.
In the examples mentioned above, when using conventional coating equipment such as a size press, blade coating means or brush, a solution of dispersion must be used which has an extremely low concentration. The reason for this is that all these known means result in relatively high absorption of liquid in the paper. The amount absorbed naturally also depends on a number of factors other than the equipment used, such as the web speed, the absorption capacity of the paper, surface roughness, etc. However, in general it is desirable to find a method of treating the paper web in which small quantities of liquid are absorbed under controllable conditions.
A method of reducing the quantity of water absorbed has been suggested for surface-sizing and surface-colouring of paper in which the paper web is first given a suitable surface uniformity and then passed over and in direct contact with a gap extending transversely over the web, to which gap the treating liquid is supplied, the entire quantity of liquid being applied on the web. According to this method the paper web completely covers the edges of the gap and no liquid may be permitted to leak outside the edges of the paper. Assuming that the gap, seen in the direction of movement of the web, is not too wide and that the web speed is not too low, the quantity of liquid applied appears to be less dependent on the absorption capacity of the paper and more dependent on the surface roughness of the paper. If the quantity of liquid to be applied is to be low the paper web must be given correspondingly high surface uniformity by means of smoothing, for instance, thus limiting the usefulness of the method since a high surface uniformity is not always desirable for other reasons. In order to obtain uniform coating all over the web, this must be free from uneven longitudinal folds or the like and must be stretched relatively tightly over the gap. This may lead to a risk of rupture in low mass per unit area papers and weak and/or absorbent papers. With high speeds, and especially when using treating liquids with relatively high viscosity, the high hydraulic pressure from the liquid rushing forward tends to lift the paper web from the edge of the gap, resulting in uncontrolled coating. This can only be compensated by still greater paper tension and thus even greater risks of unevenness. Since the liquid must be pumped with a relatively high pressure in order to achieve the requisite flow, a rupture in the web while coating in paper machines entails a risk of considerable mess since the liquid sprays out of the open gap. Furthermore, the method does not permit controlled coating in patches or points which may be desirable in some kinds of coating with very small quantities of coating agent. It is also extremely difficult to adjust a suitable quantity of liquid.
In the cases described above the treating agent is applied so that it covers the entire surface. In some cases this is not necessary or even desirable. This may apply, for instance, when moistening the paper web with water or coating it with friction-reducing agent, for instance for fluting in the manufacture of corrugated cardboard. In order to reduce the quantity of liquid applied, different methods and means are used in some cases so that the treating agent is applied in patches or points on the surface. Such coating methods may comprise spraying or splashing the coating agent in a water solution or water dispersion onto the web with the help of brushes or the like. Another method is to effect coating in patches with the help of engraved rollers. These methods have many disadvantages. Spraying often entails a risk of uneven coating since, particularly with wide webs, many spray nozzles must be placed close together, resulting in difficulties in achieving the desired uniformity. Spraying is also sensitive to draughts which may affect the distribution. Furthermore, spraying of certain treating agents is unsuitable for environmental reasons as there is a risk of the treating agent being spread through the air. The same applies to splashing by means of rotating brushes, for instance. Pointwise coating with engraved rollers is only suitable for narrow webs since larger rollers of this type result in expensive and complicated apparatus.
It has previously been suggested to coat a web of paper or cardboard with a foam produced in known manner and consisting of a liquid in which suitable treating agent has been dissolved or dispersed, and possibly added foaming agent. A foam thus manufactured in known manner is applied on the cardboard web in accordance with the suggestion with the help of a suitable applicator. By afterwards crushing the foam with the aid of a knife or some other mechanical means, a coating is obtained on said web. The advantage with this suggestion may be said to be that a better control of the coating is possible. Coating agents in the form of dispersions must be kept moving in order to prevent settling, which means that it is difficult to check the coating liquid if a rupture occurs in the paper web. One advantage of producing a coating with foam is that if a rupture occurs the supply of foam can easily be stopped by means of a valve and the machine can then easily be started again. Foam can be applied as a coating agent located upstream of a nip formed by two rotating rollers, for instance in a size press or a blade-coating machine. Although certain advantages can be achieved by such known foam-coating methods, it has been found that the liquid absorption when coating with a foam consisting of, for instance, water and size, is as great as with conventional coating methods.
It is clear from the above that there is a need for a method enabling a moving web to be coated with treating agent in a simple arrangement so that the agent is distributed in a controllable manner as uniformly as possible over the surface, while at the same time the quantity of liquid supplied is as small as possible in order to reduce drying costs after the treating process.